When a data packet network is deployed over a synchronous digital hierarchy network (as SDH or SONET) infrastructure, both TDM (Time Division Multiplex) and Data (packet) traffic are carried. In the following the synchronous digital hierarchy network will be identified with the term SDH/SONET.
In the SDH/SONET network, when it is organized in a ring configuration, one of the most common protection systems deployed is the well known MS-SPRING protection mechanism, as described in the standard ITU-T G.841, whereby the transport capacity is divided between Working and Protection capacity/channels. Every Working channel has a corresponding Protection channel to be used by the MS-SPRING protection mechanism to restore the traffic of the working channel in case of failure.
In normal non-failure conditions the Working channels carry data and TDM traffic, and the Protection channels may be used to carry the so-called low-priority “extra-traffic”.
However in case of failure, the low-priority “extra-traffic” in the protection channels is pre-empted by the MS-SPRING protection mechanism to allow restoration of the traffic of the Working channels.
Therefore this protection mechanism has the main drawback that in practice 50% of the global SDH/SONET capacity resources are wasted and typically are not used for transporting traffic, just because this traffic should have to be pre-empted in case of failure, this way reducing drastically the reliability of the transmission of the extra-traffic.
This problem is particularly important due to the strong need to transport data over SDH/SONET, and therefore the SDH/SONET capacity must be used in a more efficient way.
The same considerations are applicable to the more general situation of ring or meshed networks with other types of protection deployed, such as ring SNCP (Sub-Network Connection Protection) where a protection capacity is shared among N working capacities.